I think Luxembourg's situation is too complex to determine "most commonly spoken". How do you define that in a country where education is trilingual and Luxembourgish is the most common native language?

There genuinely isn't a lingua franca. It's not like a lot of places where there's separate language groups and then one second language everyone speaks for inter-group purposes. Basically all native Luxembourgers are trilingual, and all three get used interchangeably.

According to this: https://en.wikipedia.org/wiki/Languages_of_Luxembourg Luxembourgish is spoken by the majority of the population as a native tongue. 52% to 16% French. Definitely needs to be updated or if you want to have French as an answer use a different country.

Ok I think it's too early for me, didn't read the quiz title.... Started by putting Australia to no avail, so thought huh it must be NZ, nope... I then tried other countries and got angry that the quiz was broken.... Then realised what the title had said.. Will play again after a coffee!

The Belarussian language is not the same as Russian. The two are very similar to each other, but are still different.
If you ever visit Belarus [and I know you won't], and you call people there Russian, they will not be very happy.

Yes - I think belarus may have been a bit problematic as a country. I believe that any OFFICIAL census figures are likely to be inaccurate for political purposes. You don't dissent in belarus! The reality is that MOST people will speak some form of Rusyn (whether you want to call this Belarusian; Russian or Ruthenian) A very troubling answer fraught with political repercussions.
dave